The above-mentioned mattress generally is configured to have an interior provided with spring units formed so that a plurality of pocket coils formed so as to house coil springs is held integrally in a row in a planar shape, and an outer periphery is covered by an enclosure with favorable texture or the like. From such a configuration, when, for example, a user reclines on the mattress, the coil springs are depressed and contracted in accordance with the load (the weight of the user), and the mattress (coil springs) becomes moderately depressed. The user can therefore maintain a natural sleeping position and is able to obtain a comfortable sleep, without excessive pressure being applied to the body.
The mattress hardness (the extent of the depression) that feels comfortable for sleeping varies for each user. For example, while there are some users that prefer a mattress with comparatively hard elastic characteristics less likely to be depressed under a load, there are other users that prefer a mattress with comparatively soft elastic characteristics that is easily depressed. A variety of mattresses have conventionally been developed to meet such various user preferences. For example, FIG. 1 of Patent Document 1 discloses a mattress configured so that a plurality of pocket spring assemblies 2 having different elastic characteristics is prepared and the pocket spring assembly 2 provided with desired elastic characteristics is selected and housed in a cushion frame 1.
In recent years, rather than configuring the entire mattress to have the same elastic characteristics, there has been a demand for a mattress configured so that each portion has different elastic characteristics in accordance with the user preference. The use of such a mattress makes it possible to have a configuration in which, for example, the elastic characteristics are different at the upper body and at the lower body, enabling further improved comfort.